As a practical application of X-ray CT, there is a technique that estimates the type, an atomic number, the density, and the like of a material in a subject, which technique is known as material decomposition. In material decomposition, the fact that the interaction between the X-ray and the materials are different depending on the energy of the X-rays is used.
As a technology performing material decomposition, there is known a technology that utilizes a spectrum obtained by measuring each energy of photons that have passed through the subject and that have reached the detector. For example, there is disclosed a technique in which abundance of a material is calculated based on the average absorption coefficient of the material and an image obtained by irradiating to the subject radioactive rays of a plurality of energies.
However, in the prior art, the spectrum and the image used for the material decomposition are sometimes different from the actual spectrum of the radiation that has been transmitted through the subject or the image obtained from the spectrum. Thus, in the conventional technique in which material decomposition is performed based on the spectrum and the image, it was difficult to accurately perform material decomposition.